We tend to honor the Martin Luther King Jr. we want to honor, not the Martin Luther King Jr. who actually existed.

We forget the King who at the time of his ministry was labeled an "extremist," who explicitly called out "moderates" for urging African Americans to slow down their march to justice, who quite brilliantly used the American creed as a seedbed for searing criticisms of the United States as it existed.

The postponement of the planned ceremonies dedicating the new memorial to King did not come in time to stop the tributes from flowing in advance. This was a blessing. Debating the meaning of King's legacy is one of the best ways of ensuring it endures - although some will always try to domesticate him into a self-help lecturer who'd be welcomed at the local Chamber of Commerce or even a Christian Coalition meeting.

That we have failed to live up to King's calls for economic justice is one telltale sign of our tendency to hear King's prophetic voice selectively. But selectively hearing him is better than not listening at all, as long as it doesn't lead to a distortion of what he believed.

One of the many things King understood was the always incipient radicalism of the American idea. At a time when paying homage to our nation's origins seems far more a habit of the Tea Party than of progressives, King, like Abraham Lincoln before him, threw our founding documents in our faces and challenged us to take them seriously.

His "I Have a Dream" speech was an extended and impassioned essay on the American promise. The civil rights movement's demands, he insisted, arose from American history's own vows.

"When the architects of our republic wrote the magnificent words of the Constitution and the Declaration of Independence," King proclaimed, "they were signing a promissory note to which every American was to fall heir. This note was a promise that all men, yes, black men as well as white men, would be guaranteed the 'unalienable Rights' of 'Life, Liberty and the pursuit of Happiness.'"

One of the most dramatic moments in the speech came next. "It is obvious today that America has defaulted on this promissory note, insofar as her citizens of color are concerned," King said. "Instead of honoring this sacred obligation, America has given the Negro people a bad check, a check which has come back marked 'insufficient funds.'"

This is the wonderful paradox of King: A Christian preacher, he understood the power of rooting arguments in a tradition. But this did not make those arguments any less radical. His emphasis was on those words "insufficient funds," on our sins against our own claims.

This focus on calling out injustice - pointedly, heatedly, sometimes angrily - is what the people of King's time, friend and foe alike, heard. It made many moderates (and so-called moderates) decidedly uncomfortable.

King also declared himself "gravely disappointed with the white moderate" who, he feared, was "more devoted to 'order' than to justice."

And recall King's response to being accused of extremism. Though "initially disappointed at being categorized as an extremist," he wrote, "as I continued to think about the matter I gradually gained a measure of satisfaction from the label." We have rendered King safe so we can honor him. But we should honor him because he did not play it safe. He urged us to break loose from "the paralyzing chains of conformity." Good advice in every generation - and hard advice, too.